A few days ago I mentioned problems with using polygonal rifling and soft lead bullets, in my post Glocks and Safety.
Since this issue is a common question (in fact I originally wrote this in response to an emailed question and a comment on the post), I thought I'd explain it here.
First we neEd to define what polygonal rifling is, and why it's different from "conventional" riflng.
There's a lot of ways to make a barrel, from rolling steel into multiple layers of tubing over a mandrel (a long central form) and then forging the tubing together around it; doing the same with wire or multiple rods of steel spiral wrapped around a mandrel; or boring a hole in a long piece of bar stock and lathe turning it; to combinations of the above (and others as well).
Most modern barrels are turned from bar stock, and have a long hole bored through them; thus "the bore" of the barrel. Of course that bored hole is smoothe (or at least relatively smoothe), and now we need to rifle it.
There are a few different way to rifle the bore. You can cut individual spiral grooves (as many as 9 in conventional designs, and potentially hundreds with microrifling) with a single cutter. You can cut all the grooves at once by pulling something called a broach through the bore. You can also press the rifling into position using pressure (with or without heat) with something called a button. Finally, you can form the rifling into a mandrel, and then forge the barrel around it by hammering on it (again with or without heat).
Cut, broached, and button rifles barrels all cut or press grooves (called appropriately enough grooves) into the metal of the bore, leaving raised spaces between them called lands. The initial reason for these however wasn't for accuracy, it was to leave a place for the fouling produced by dead soft lead balls and dirty black powder to go so that a gun could be fired more times without cleaning; and without dangerous perssure and obstructions building up in the bore. Those obstructions and excess friction from buildup could cause massive overpressure and make a gun explode.
The oldest form of rifling for accuracy used the last method I listed above. By hot forging spirally wound rods around a mandrel which was hexagon shaped, and twisted along it's length, the sides and corners of the mandrel were pressed into the shape of the bore. That was the worlds first polygonal rifling; thus called because instead of lands and gooves, the bore had sides and corners like a hexagon, which is of course a polygon (a multi sided shape).
Unfortnuately this early method was difficult to manufacture barrels with; and the barrels produced often had flaws that could cause explosions. Additionally the dirty propellants, and poor quality of lead alloys used in projectiles of the time would cause excessive fouling.
Thus back to the cut, broach, and button rifled barrel, for about the next 600 years.
Coming into the 60's and 70's, manufacturing technology had advanced... let's just say a lot; and so had the bullets we were shooting through our guns.
The "best" form of rifling is cut rifling with a single cutter; unfortunately it's slow, expensive, and requires very expert machine work to manufacture properly. The reason it's considered best, is because it puts very little stress on the barrel while cutting, which produces a more even, consistent cut; with even distributions of stresses around the barrel. This produces more consistent barrel harmonics, and thus better accuracy and precision.
Broach cut and button rifled barrels both exert a fair bit of pressure on the barrel while forming, and that pressure can sometimes be inconsistent; introducing non-uniform stresses into the barrel, and less consistent barrel harmonics.
Of course most manufactures use those because they are faster, and less expensive.
Enter once again, polygonal rifling. Manufacturing capabilities, and improved jacketed bullets have advanced to the point where it is once again viable; and it has some advantages.
There are a few good reasons why manufacturers use polygonal rifling:
First, it's very easy, fast, and inexpensive to manufacture by hammer forging. A mandrel with the proper shape is inserted into the bore, and the barrel is cold forged around the form; pressing the polygon shape into the bore.
This produces a barrel with extremely consistent rifling, that's VERY smoothe. Importantly it also produces a barrel with exactly even stresses along the rifling; which is stronger, and more consistent than either broached or button rifled barrels (though not as much as the single cut rifling).
As a side effect, the very smoothe polygonal bore, with no sharp cuts to disturb it; produces a very good pressure seal against a fired bullet. This results in higher velocity, and can aid in accuracy and precision (along with the improved barrel harmonics).
There are two prolbems with polygonal rifling though. Because of that great pressure seal I mentioned; polygonally rifled barrels exhibit a greater tendency towards overpressure; just as they did when the rifling method was first developed. Again, just as when first developed, because there are no grooves to press the fouling into, the barrels have and a greater tendency towards fouling and leading.
The reason why there have been problems with lead bullets in the past, and why manufacturers recommend against it is two fold:
First, lead is permanently deforming. It is malleable but not very elastic, so when it takes a shape, it stays that way.
In a conventionally rifled barrel, this isn't as much of a problem. The lead is displaced from the lands into the grooves, and all is well. In a polygonal barrel, this can't happen, because there are no grooves to displace into. The lead is forced into the shape,and the only way it can move is forward and backward. This is more difficult than the sideways displacement into conventional rifling grooves, and increases pressures; anywhere from a little bit, to a heck of a lot depending on the alloy, and the tolerance from bullet to bore (.450 bullet in a .451 bore is fine, .453 bullet in a .450 bore and we've got a problem).
Anyway, none of these would be a serious problem with a relatively hard bullet, because the bores are made to have enough tolerance for the lead to squish sideways into the polygonal vertices (corners) somewhat.
Thus we get to the second problem causing property of lead: It flows and deposits under even hand friction and pressure. When under great pressure, heat, and velocity, lead will plate the bore of a polygonally rifled barrel quite handily. Of course it will also plate the bore of a conventional rifled barrel, but most of the leading is in the grooves, which have a greater tolerance for these things.
This lead plating builds up until the vertices are filled, at which point the pressure starts to increase greatly. If the plating builds up enough, the bullet will either be obstructed causing massive overpressure (and possibly a blown or bulged barrel); or if the tolerances are somewhat greater (enough to avoid overpressure), the barrel will effectively become a smoothebore, radically reducing accuracy and precision.
This is why manufacturers of polygonally barreled guns recommend you use only jacketed or solid copper bullets in their guns. Copper is quite elastic, and it will spring back when slightly deformed. It is also far harder on the surface then lead is so the amount of plating is greatly reduced. Finally, the tolerances on jacketed bullets are usually tighter than withsoftcast lead.
So the problems DO exist with jacketed bullets, it just takes a lot longer for them to become serious.
Of course I keep mentioning soft cast lead. The reason is, most commercial lead bullets are soft cast (or soft swaged); meaning they are well, soft. You can easily put a big gouge in one with your thumbnail. The good news though, is that if you use a harder alloy (more antimony and a bit more tin plus maybe some vanadium), and cast into a tempering bucket (hard casting); you generally won't have the same kind of leading problems. So if you cast and load your own hardcast, or buy bullets that are advertised as hardcast, you should be just fine.
Unfortunately I don't know of any major commercial manufacturers who load hardcast lead bullets (the economics of them dont workout for the majors); so you are limited to either specialty manufactures like buffalo bore; or to handloading.
Of course Glock, HK, Kahr, and Marlin (the major manufactureres most associated with polygonal rifling) will still void your warranty if you use lead bullets (hard OR soft); but they do that with handloads anyway so it's not like you're losing out much.
